Apple has released iOS 6.1.4 specifically for the iPhone 5, which. As Dan Moren from Macworld astutely points out, Apple's somewhat veiled language in describing the upgrade - "updated audio profile for speakerphone" - may or may not mean that your voice will sound louder or clearer over speakerphone.
It's only 11.5 MB, though, so it's worth a shot - at least if you're a heavy speakerphone user.